Hi hopefully I’ve managed to post in correct section!
I have a 12ft kayak and would like to mount it on my Ford Focus roof (2015) with some already installed Halfords roof bars that’ll take about 70kg.
The kayak weighs 32kg. Will I get away with just tieing two of the standard tie down ropes (not ratchets) one on each bar as many YouTube videos show..but not using front and back supports as i have seen in some pics? Want it to be safe but new to all of this.
Thanks in advance!

If it's plastic, put it upside-down on the roof bars and try to get the cockpit caught between them to stop it sliding forward. Use straps which are long enough to pass over the boat, round the bar on the other side and back over the boat again to be tied down. Strap it down firmly - but not too tight - Just tight enough to make sure it doesn't move when you push and pull on the bow. If it's windy - tie-downs front and rear are a good idea.
Strapping to roof bars, I usually make sure I've got a couple of turns of the strap round the post and have the buckle of the strap coming down over the boat to make it easier to tighten. Stop after a few miles (or before you get onto a motorway) and check that everything is still secure.
If it was a composite boat I'd suggest you treat it to a decent set of J bar cradles - it's well worth it. :-) but at 32kg, I'm guessing it's plastic.

J bars or cradles are a good investment and they do make it much easier to secure the boat on the roof, but for a short(ish) plastic boat they're not essential.

Can you confirm that weight? 32 kg = 70.5 pounds. That is more than double what I would expect from a 12 foot long kayak. It would also, if accurate, be a weight that is liable to cause injury to you when lifting the kayak onto a roof rack. I see that some polyethylene kayaks like the Riot Enduro 12 are 26 kg, but 32 kg seems very high.
Nick.

If the roofbars fail and the tie downs don't you could still end up with the boat down the side of the car and interfering with the wheels which doesn't bear thinking about!
Tie downs are to stop the boat going backwards or forwards only and if the rack or bars fail the tie downs will then be slack and not doing what they were originally intended to do. In a past life I attended an incident on the motorway where the front J bar failed due to rusted bolts, once the J bar collapsed the tie down was lose and the wind lifted the boat which flapped from side to side and because the tie down was now lose it snapped due to the forces causing it to jerk, so the boat then tried over flip over backwards and snapped at the rear strap and J bar and the front section landed behind the car. All happened in seconds. Thankfully no one was hurt.
In a case of a sudden stop yes the tie downs can stop the boat flying forwards.
I use J bars but don't simply trust in them. Yes my straps go around the top bar but I secure them around the car roof rails in such a way that the straps actually pull both the j bars and roof rack down onto the roof rails. I also use 2 straps on the front as this is subject to the strongest forces.

it is not just static load which has to be taken by the roof bars (AND the attachments which fix the roof bars to the vehicle) but the dynamic load imposed by the wind resistance as you hurtle up the motorway. The force imparted on the kayak (and hence whatever it is tied to) as your vehicle moves through the air at 70mph is far in excess of the 32kg downward weight of the kayak. The front tie down line is especially important to avoid the upward wind force on the kayak trying to fold it back and rip your roof rack off. Have you ever seen somebody trying to get a roof rack load of plywood home from the DIY store only to see it fold and snap like cardboard!!
Use a front line (Thule 'Quick Pulls' are really good), put a cockpit cover on and get rid of any protrusions (deck bags/spare paddles - yes I've seen it!!). Sometimes you can't avoid side winds but luckily we all drive in the same direction as the kayak is facing so we already unconsciously reduce wind resistance.

If you've not already done so, it's worth a look in the Almanac / Equipment page where there are some links to earlier discussions on this subject. Be wary of Thule's J cradles. Their bars and fitting kits are fine. KCS and Karitek make good cradles.